Coatings are used in a wide variety of applications to protect underlying structures from their environments. For example, coatings can be used to resist corrosion, to provide thermal insulation, to prevent mechanical damage, to reduce radar observability, or to protect from lightning strikes. Coatings include, for example, paint and polymer-based appliqués, which are being considered by military and commercial aviation operators as an alternative to paint. In aviation, for example, the thickness of coatings such as those used on airplane propellers and helicopter blades are required to be maintained within a certain range that is thick enough to allow for corrosion resistance, but thin enough to allow for the use of de-icing technology.
As inadequate coating can reduce the protection offered to underlying structures, and excessive coating can be expensive, particularly where hi-tech coatings are concerned, such as those coatings with low observable characteristics favorable for use on stealth aircraft. An extra few thousandths of an inch of unnecessary coating thickness can add significant expense to the manufacturing process for an aircraft part. Excess coating can also lead to unnecessary added weight or, in the case of airplane propellers or helicopter blades, an unbalanced rotating assembly. Because a propeller or helicopter blade is sensitive to balance, the stability of the rotating mass is subject to the consistency of the coating thickness applied throughout its surface area.
Measuring coating thickness, therefore, is desirable and often critical in order to determine proper coating thickness particularly of curved surfaces such as, for example, those of airplane propellers or helicopter blades. Additionally, in applications relating to measuring the thickness of hi-tech coatings used in aircraft and spacecraft, fast, accurate measurements are also critical.
Microwaves can be used to measure the thickness of paint and other coatings, as described in U.S. Pat. No. 7,339,382, to Bray et al. for “Apparatuses and Methods for Nondestructive Microwave Measurement of Dry and Wet Film Thickness,” which is hereby incorporated by reference. Bray describes the use of microwaves for measuring film thickness by comparing properties of reflected waves from a sample to properties of reflected waves calibrated by passing the waves through films of known thicknesses.
Prior art microwave thickness measurement methods take advantage of the way in which incident radiation interacts with layered dielectric media. Microwave energy is directed from a microwave source toward a coating, such as paint or appliqué, over a target substrate. A portion of the microwave energy is reflected from the target surface underneath the coating to be measured. The thickness and dielectric properties of the coating through which the microwaves pass affect the properties of the microwaves. The reflected microwaves combine with the primary waves to form a standing wave pattern. Preferred embodiments determine the quadrature components of a reflected wave, which quadrature components are then correlated to coating thickness, preferably using correlation data determined from calibration standards.
As the dielectric characteristics under the aperture of the antenna change, the reflected signal changes. On an aircraft, the layers of materials over bright aluminum typically include an anti-corrosion coating, a primer coating, and a topcoat or an appliqué. These dielectric layers affect the transmission characteristics of the microwave signal in a relatively constant fashion on the skin of the aircraft.
FIG. 1 shows a prior art device used to measure coating or film thickness on a substrate 30. A microwave oscillator/transmitter 12 is the source of microwave energy for the device 10 and should generally be matched to a waveguide 16 in frequency. A reflected energy separator 14, such as a three-port circulator, can be used to separate reflected microwave signal from the incident microwave signal. A circulator typically uses a ferrite material biased by a DC magnetic field, which routes microwaves differently depending on the direction of propagation. This allows the reflected waves, traveling back, to be separated from the primary waves, traveling forward. The reflected wave quadrature components can thus be measured directly. In-phase detector diode 18 and quadrature diode 19 can sense radiated microwave energy and convert it into an electrical signal. The thickness of the coating 26 on a substrate 30 will be proportional to the phase of the reflected microwave signal.
As discussed in greater detail below, the phase of the reflected signal will be dependent on the analog outputs of these two diodes in quadrature with each other. The standoff distance 24 (the distance between the microwave sensor/detector and the target) must be maintained at a constant value because variations in the distance between the microwave sensor (indicated by dashed line 20) and the target surface (indicated by dashed line 22) can cause significant changes in measurement results, as described below. As used herein, the term “standoff distance” will refer to the distance between the microwave sensor/detector and the target.
Prior art measurement methods and devices suffer from a number of shortcomings. Such devices are typically large and bulky and are accordingly difficult to use in the field for spot checking thicknesses. Typically, truck assemblies or robotic control are required to maintain constant liftoff/standoff distances. Further, existing devices are very difficult to use on sharply curved surfaces such as helicopter blades and airplane propellers.
Accordingly, what is needed is an improved method and apparatus for non-destructive measurements of coating thicknesses that is both portable and easy to use on curved surfaces.